The Collaborative International Dictionary of English v.0.48:

grease gun \grease gun\, grease-gun \grease-gun\n.
   A device held in the hand, having a supply of grease and
   attached to a reservoir of pressurized air, used to force
   grease between adjacent moving parts of a machine, especially
   in the bearings of motor vehicles.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

grease-gun
    n 1: a hand-operated pump that resembles a revolver; forces
         grease into parts of a machine [syn: grease-gun, gun]